vCJD may lurk in more people than realised

The deadly human form of mad cow disease, vCJD, may have infected far more people than previously thought, suggests a new study.

The assumption that most people are genetically shielded from the devastating disease could be wrong, said the research published on Friday. But it cautions that the evidence for this remains sketchy.

Variant Creutzfelt-Jakob disease (vCJD) is linked to eating meat infected with bovine spongiform encephalopathy (BSE), also known as mad-cow disease. A rogue version of a prion protein proliferates in the brain, leading to distressing mental deterioration, loss of motor control, and eventually death.

After vCJD was first identified in March 1996, some experts calculated it could inflict a death toll in the tens of thousands, especially in the UK, where the outbreak began. But these calculations were swiftly revised downwards to a few hundred or even fewer when it was realised that the toll was rising far slower than expected.

Advertisement

Key variation

At present, the UK has recorded 161 definite and probable cases of vCJD, six of whom are still alive. One reason for optimism about the potential extent of the vCJD epidemic has been the assumption that it is genetic.

All of the deaths have occurred among people with a so-called “MM variation” in part of the prion protein gene, called PRNP, located on chromosome 20. In the white British population, 42% of people have the MM variant.

The rest of the white population have different types – 47% have the MV variation while the remaining 11% are VV. The fact that no MV or VV cases had arisen led many to believe that this was a protection against the rogue protein.

Two out of three

But the new study, which appears in the British Medical Journal, places a cloud of doubt over this assumption.

Researchers led by James Ironside at the University of Edinburgh, UK, carried out a DNA analysis of three appendix tissue samples found to carry the mutant prion protein.

The tissues were part of a vast earlier study in which UK labs screened 12,600 appendices and tonsils for the protein in order to get an idea of the spread of vCJD.

Ironside’s team say they were extremely surprised to find that two out of their three samples, which tested positive for vCJD, came from people with the VV variant. Neither individual, both aged in their twenties at time of surgery between 1996 and 1999, has the symptoms of vCJD.

Incubation time

But the paper warns against dramatism. It notes that only these two VV samples have so far been identified, and just because a VV individual has the protein does not mean that he or she will go on to develop vCJD.

On the other hand, no one knows how long it takes for vCJD to incubate, which raises the possibility that VV individuals may fall sick years from now. In classic Creutzfeldt-Jakob disease, which occurs in older people, this can take up to 30 years.

And another, as-yet unquantifiable, risk is that VV individuals with the prion may unwittingly pass it on to others through blood donations. In 2004, a person with the MV gene variant was found to be infected with vCJD, but again this individual had no clinical disease.

“There are compelling reasons why health officials should take notice,” said two Canadian specialists, Kumanan Wilson and Maura Ricketts, in a commentary accompanying the latest paper.

“It is conceivable that, having jumped the species barrier (from cows to humans), transmission of the prion within the species becomes easier.”